The presynaptic, antidepressant-sensitive serotonin (5-HT) transporter (SERT) is the major mechanism for 5-HT inactivation and recycling in the brain and periphery. In accord with its powerful control over 5-HT signaling and homeostasis, SERT synthesis, trafficking and activity are highly regulated by multiple signaling pathways. Alterations in SERT expression, function or regulation have been implicated in multiple neuropsychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, and autism, though much evidence is indirect, owing in large part to a lack of opportunities to dissect SERT regulation and dysregulation in vivo. Over the last period of support for this project, we demonstrated that an inflammatory cytokine pathway linked to IL-1R/p33? MAPK activation rapidly translates immune system activation to elevated SERT activity, increased 5-HT clearance and altered behavior. Remarkably, this pathway appears to be unable to modulate activity of several autism-associated SERT coding variants, including the hyperfunctional variant Ala56, supporting the idea that a dual analysis of both heritable SERT mutations and cytokine-triggered regulatory pathways can advance our understanding of both fundamental SERT regulation and the networks that drive maladaptive SERT states associated with mental illness. We hypothesize that SERT Ala56 constitutively imposes ?outward- facing? conformations (SERT*) that are normally occupied transiently during the course of normal, presynaptic IL-1R/p38 MAPK regulation. Furthermore, we propose that stabilization of SERT* by the Ala56 mutation throughout life drives disease-associated changes in 5-HT signaling and, more pertinent to the current proposal, provides a unique opportunity to capture transient molecular alterations, interactions and functional states that are otherwise difficult to study. In pursuit of this idea, we generated SERT Ala56 KI mice, and have demonstrated that these animals display autism-like behavioral features, as well as biochemical and functional changes consistent with constitutive IL-1R/p38 MAPK stimulation. To dissect the requirements for immune activation pathways that normally activate wildtype SERT and that contribute to the constitutive hyperactivity of SERT Ala56, we generated mice that afford conditional suppression of either p38? MAPK or its upstream activator, the IL-1R. We also generated mice that prevent or mimic phosphorylation at a key site linked to SERT conformational stabilization and trafficking-independent SERT stimulation. Finally, we appropriate a newly developed, CNS penetrant and isoform-selective, p38? MAPK antagonist, whose use can bypass the potential confounds of developmental p38? MAPK deletion. With these tools we interrogate changes in native SERT phosphorylation states, SERT interacting proteins (SIPs), and functional states, clarifying the relationship between SERT* states of SERT Ala56 and immune-activated transporters. Altogether, our efforts provide an exciting opportunity to gain a deeper understanding of the mechanisms that govern SERT regulation while illuminating broader networks that define risk for brain disorders linked to perturbed 5-HT signaling.